# Whatbs the meaning of variables in the computed results

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 May 26, 2005, 03:15 Hi, All I'm a new user of Ope #1 New Member   MaJianfeng Join Date: Mar 2009 Posts: 8 Rep Power: 9 Hi, All I'm a new user of OpenFoam. I just found her several days ago and was interested in its open and robust attributes. After plays with it, i think it has a bright promising if it's help documentation can be strengthened. Now, comes my question: I run the LES case:channel395 and get several results written as files in different directory,e.g. in directory 1000, there are files of P,U,u_0,k,.....etc. My exact question is what's the meaning of U_0,Umean,R,Bmean. Does Umean means the average U with time?? Can anybody help me??

 May 26, 2005, 04:25 Umean is a running average of #2 Senior Member   Gavin Tabor Join Date: Mar 2009 Posts: 181 Rep Power: 9 Umean is a running average of the velocity field and R is a running average of the 2nd moments (velocity variance av(u'u'). Bmean is a running average of the SGS stress tensor. By running average I mean a continuously-updated time average of the quantity. Most of this is in createAverages.H, which is fairly easy to read once you get used to the syntax. I actually can't find where U_0 is being defined : I'm wondering if its redundant. I'll keep looking. Gavin

 May 26, 2005, 05:06 U_0 and the other *_0 fields a #3 Senior Member   Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 13 U_0 and the other *_0 fields are the old time values (i.e. timestep previous to U). They are necessary to restart calculations that use second order accurate schemes in time like Crank-Nicholson. For the rest,"p" for a incompressible code is pressure devided by density. For a compressible code it is just the pressure (same for mean quantities). "pPrime2Mean" is the mean of the square of the deviatoric part (p-pMean) of the pressure and is a good indicator for noise. "nuSgs "is the subgrid-scale viscosity and "k" is the SGS turbulent kinetic energy. "nuTilda" is a working SGS viscosity that is only used with the Spalart-allmaras model. Have I missed anything? arunsmec likes this.

 January 29, 2007, 13:55 Hello Does anyone know what #4 Senior Member     Jose Luis Santos Join Date: Mar 2009 Location: Portugal Posts: 215 Rep Power: 10 Hello Does anyone know what is the definition of the 9 components of Reynolds stress R in OpenFOAM? Thanks!

 September 12, 2007, 17:24 Hi all, I was looking at w #5 Senior Member   kumar Join Date: Mar 2009 Posts: 112 Rep Power: 9 Hi all, I was looking at what the components of R is and how they are calculated in channelOodles. In createAverages.H, R is created as a symmetric tensor. so the 6 components of R written out by the program are Rxx,Rxy,Rxz,Ryy,Ryz and Rzz. As Dr. Tabor remarks above , R represents the variance of velocity thus the components of R should ; Rxx=ave(u'u') , Rxy=ave(u'v') , Rxz=ave(u',w'), Ryy=ave(v'v') , Ryz=ave(v'w') and Rzz=ave(w'w'). so the dimensions of R should be [0 2 -2 0 0 0 0] . But if you look at the output file of R generated by the solver (say channelOodles.C) , then its dimension is [ 0 1 -1 0 0 0 0]. Is this correct? Also in createAverages.H, R is calculated as sqr(U)-sqr(UmeanPtr()) . thus R seems to be a function of velocity squared and should have dimension of m^2/s^2. and can someone tell me how to get u'u' from sqr(U) - sqr(Umean) . It seems to me this should be square(U - Umean) Thanks a lot kumar

 October 15, 2007, 07:59 Hi Kumar, About your last q #6 Member   hadi tartoussi Join Date: Mar 2009 Location: paris Posts: 48 Rep Power: 9 Hi Kumar, About your last question i can tell you the following (PLEASE SOMEONE CORRECT ME IF I AM WRONG): -R is a running average of u'u' so R==<(u-umean)(u-umean)> =<(u-umean)^2> = = - 2 + = -2.Umean + Umean^2 = -2.Umean^2 + Umean^2 = - Umean^2. Cheers, Hadi

 October 15, 2007, 08:01 Hi Kumar, About your last q #7 Member   hadi tartoussi Join Date: Mar 2009 Location: paris Posts: 48 Rep Power: 9 Hi Kumar, About your last question i can tell you the following (PLEASE SOMEONE CORRECT ME IF I AM WRONG): -R is a running average of u'u' so R==<(u-umean)(u-umean)> =<(u-umean)^2> = = - 2 + = -2.Umean + Umean^2 = -2.Umean^2 + Umean^2 = - Umean^2. Cheers, Hadi

 June 13, 2010, 22:46 #8 Senior Member   Jiang Join Date: Oct 2009 Location: Japan Posts: 186 Rep Power: 9 Very good! Thanks for engene, kumar and hadi's explanation. hadi's explanation is especially excellent. This thread shouldn't be forgot.

November 12, 2012, 02:52
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Aru
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Quote:
 Originally Posted by eugene U_0 and the other *_0 fields are the old time values (i.e. timestep previous to U). They are necessary to restart calculations that use second order accurate schemes in time like Crank-Nicholson.
If it is necessary only for restarting calculations, why we have to store this data? I am doing LES and is experiencing difficulty with disk space. If I dont save this *_0 fields, I can save about 10 GB space. Would you recommend removing these fields?

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